1. Field of the Invention
The present invention relates to a semiconductor laser driving technique and, more particularly, to a driving technique in a low temperature range of a nitride semiconductor laser.
2. Description of Related Art
In recent years, a wideband gap semiconductor laser diode made of a material such as gallium nitride (GaN), which is the material for a blue laser is being used more and more. Such a nitride semiconductor laser characterized by a short wavelength is widely applied to a pickup device for writing/reading data to/from a next-generation DVD (Digital Versatile Disc), a display, a printer, medical equipment, and the like.
Generally, the brightness of a semiconductor laser is determined according to the amount of current flowing in the device. Therefore, to make a semiconductor laser emit light with a desired brightness, it is necessary to perform constant current driving by supplying a current from a driving circuit and according to the desired brightness (refer to Japanese Patent Laid-Open No. H5-259544 and Japanese Utility Model Laid-Open No. S63-29968).
Although the properties of an arsenic (As)-based or phosphorus (P)-based semiconductor laser emitting red light or near infrared light improve as the temperature decreases, the nitride semiconductor laser has a problem in that its properties deteriorate as the temperature decreases. This problem is caused by a large band gap and a deep impurity level used for doping of the nitride semiconductor. Specifically, since the impurity level is deep, the activation rate of carries (holes in the case of a nitride semiconductor) is low even at room temperature. When the temperature further decreases, the carrier concentration becomes lower. Generally, the electric conductivity of a semiconductor is determined by the product of the carrier mobility and carrier concentration. When the carrier concentration decreases at the time of low temperature, device resistance increases. Further, in a junction region in a diode, the carriers on the side where the decrease in the carrier concentration is larger (a P-type region in the nitride semiconductor laser) tend to deplete, so that carriers of the opposite polarity (electrons in the nitride semiconductor laser) move over the junction region to the side where the carrier concentration is low (a P-type region in the nitride semiconductor laser). The carriers injected to the opposite polarity region where resistance became higher cause energy obtained from an electric field generated due to the high resistance release in the form of a point defect, and it causes problems such as failure in the device and deterioration in reliability such as device life span.
Generally, the brightness of a semiconductor laser is controlled by current injection control using the driving circuit as described above. Consequently, when a current which is the same as that at room temperature is supplied to a semiconductor laser in which a resistance value increases at a low temperature, a voltage applied to the device becomes high at the low temperature. In some cases, this negative influence is exerted on the device characteristics, the life span, and the like.
It is expected that driving of a semiconductor laser, particularly in consumer products, is started in a low-temperature environment. Considering that next-generation DVDs on which nitride semiconductor lasers are mounted and the like are being widely sold and used in the future, it is necessary to enhance circuit protection of a semiconductor laser.